"The Department of Justice takes seriously reports of any data breach, particularly those involving personally identifiable or financial information, and looks into allegations that are brought to its attention,'' Holder told the Senate Judiciary Committee.

"While we generally do not discuss specific matters under investigation, I can confirm the department is investigating the breach involving the U.S. retailer, Target. And we are committed to working to find not only the perpetrators of these sorts of data breaches – but also any individuals and groups who exploit that data via credit card fraud.''

Hackers stole about 40 million debit and credit card numbers in the pre-Christmas data heist. They also took personal information — including email addresses, phone numbers, names and home addresses — of another 70 million people.

Minneapolis-based Target says it is working with the Secret Service and the Justice Department in response to the data breach.